Numerous studies have demonstrated that a variety of dietary constituents inhibit chemically induced tumorigenesis in rodents, including DMBA induced mammary tumors. The many steps between exposure toa procarcinogen and the transformation of a normal cell to a cancer cell begin with the activation of the procarcinogen to genotoxic forms. For the arylhydrocarbons (AH), this process is initiated by the AH receptor (AhR), a cytosolic transcriptionfactor. Natural endogenous or exogenous ligands of the AhR have been postulated but remain, for the most part, unidentified. We identified several dietary polyphenolic compounds are natural ligands of the AhR. We found curcumin is a ligand of the AhR and an inhibitor of cytochrome P450 1A1 in MCF-7 human breast cancer cells and Diosmin and diosmetin are agonists of the AhR and causing an increase in CYP1A1 mRNA. We further found that diosmetin, but not diosmin, was inhibitory to CYP1A1 activity. The result was that diosmetin inhibited adduct formation and DMBA induced cytotoxicity, while diosmin stimulated both parameters. The flavonoid galangin is an inhibitor of DMBA metabolism and an agonist/antagonist of the AhR in MCF-7 cells. The dietary flavonols, quercetin and kaempferol are ligands of the AhR that differentially affect CYP1A1 transcription. The AhR also regulates the transcription of a number of Phase II enzymes. The effect of detoxification mechanisms by dietary flavonoids are under our current investigation. - Carcinogen, Flavonoids, AhR ligands,